Chemistry Reference
In-Depth Information
(1997) studied the impact of NaCl on the properties of acid casein gels. They
found that the pH at gelation was lower,
5.0, in gels made with added NaCl
than in gels made without added NaCl, pH
5.1.
Low-methoxyl pectin is often used as a stabilizer in acid milk gel
systems. Harte et al. (2007) proposed that during the acidification of milk,
the release of Ca
2+
arising from the solubilization of CCP induces the forma-
tion of pectin-pectin complexes, and at lower pH values these complexes
interact with the casein particles. For acid casein gels made in the absence of
Ca ions, a substantial reduction in the storage modulus was detected at pectin
concentrations as low as 0.01-0.02% (w/v) and a significant increase in gela-
tion time at pectin concentrations
0.05% (w/v) (Matia-Merino et al., 2004).
Complete inhibition of acid-induced gelation of casein was noted at
0.8%
(w/v) pectin. Addition of Ca at low pectin contents (
<
0.2%) reduced the
modulus of acid milk gels but there was a large increase in the storage
modulus at higher levels of pectin (
0.2%, w/v).
9.7.3.
Heat-Induced Whey Protein Gels
Salts have a major effect on the type, as well as the mechanical/sensory
properties, of whey protein gels formed as a result of heat treatment. It is
generally recognized that the addition of CaCl
2
to dialysed samples of whey
protein concentrate (WPC) or whey protein isolate (WPI) results in an
increase in gel strength. Above a level of 10-20 mM CaCl
2
gel firmness starts
to decrease (Schmidt et al., 1979; Kuhn and Foegeding, 1991). It has been
speculated that excessive Ca causes rapid protein aggregation (due to
decreased protein stability), which limits protein unfolding and network
formation (Mangino, 1992). Caussin et al. (2003) reported that the addition
of Ca to whey proteins resulted in the formation of very large protein
aggregates during heating. Most commercially available WPC products
probably have a Ca content that is greater than that required for optimal
gel strength (Mangino, 1992). There is considerable variability in the thermal
aggregation behaviour of commercial whey products and some of these
differences could be removed by dialysis of these samples to a common
ionic strength (McPhail and Holt, 1999). The concentrations of divalent
cations are higher in WPC made from cheese whey than in WPC made
from acid whey and these cations are not easily removed by dialysis, suggest-
ing some binding by the whey proteins (Havea et al., 2001). Presumably,
membrane filtration of acid whey WPC at low pH values resulted in its
greater demineralization. WPC made from acid whey has superior heat
gelling properties than WPC made from rennet-coagulated cheese whey
(Veith and Reynolds, 2004). These differences could be due to absence of
GMP and the low Ca concentration in acid whey WPC.
